Process for producing a wear-resistant surface on a workpiece

ABSTRACT

A process and apparatus for galvanic deposition on a workpiece, particularly the inner surface of trochoidal engine housings, of a metal matrix containing dispersed particles, for the purpose of improving wear resistance of highly stressed surfaces.

United States Patent Stephan et al. 1 Feb. 8, 1972 [54] PROCESS FOR PRODUCING A WEAR- I RESISTANT SURFACE ON A R f r nces Cited WORKPIECE UNITED STATES PATENTS 1 lnvemom Gerhard Stephan, "eilbwnn/Na Heinz 3,488,263 1/1970 Schmidt ..204/38 R "first, Nsckarsulm; Gunter Law, Heil- 3,298,802 l/l967 Oderkerken ...204/32 R Alfred pp Neckarsulm, all 3,514,389 5/1970 Stephan et al. 204/237 of Germany 3,449,223 6/1969 Odekerken....... 204/38 R I73] Assignees: NSU Motorenwerke Aktiengesellschaft, 25 x f f Lmda 21848391 8/1958 Fahnoe et al ..204/38 13 milcnsec, (icrmany [ZZI Filed: Sept. 30, 1969 Primary Examiner-John H. Mack Assistant Examiner-R. L. Andrews [21 1 Appl' 871338 Att0meyRaymond P. Wallace and Victor D. Behn Related US. Application Data 57] ABSTRACT [62] 8 Sept 1968 A process and apparatus for galvanic deposition on a work- 8 piece, particularly the inner surface of trochoidal engine housings, of a metal matrix containing dispersed particles, for the 52] us. 01 ..204/3s B, 204/16, 204 40, purpose of improving wear resistance of highly Stressed Sup 204/ 1 81 faces [51] Int. Cl. ..C23t 17/00, C23b 13/00, C23b 5/50 [58] Field of Search ..204/38 RB, 38 B, 16, 40, 181 5 Claims, 4 Drawing Figures PROCESS FOR PRODUCING A WEAR-RESISTANT SURFACE ON A 'WORKPIECE Cross-References to Related Applications This application is a division of copending application Ser. No. 758,017, filed Sept. 6, 1968 and now US. Pat. No. 3,514,389.

BACKGROUND OF THE INVENTION The inner surface of the peripheral housing of a trochoidal engine is continuously swept by the sealing elements of the rotor. This action results in wear of the housing surface, and particularly where the housing is formed of a light metal such as aluminum or its alloys the wear may result in some loss of sealing action, with consequent leakage of gas and loss of power. It is therefore advantageous to provide this inner surface with a hard, smooth, wear-resistance coating.

Galvanic or electrolytic processes for plating a hardmetal onto a softer one are well know. It is also known to produce a hard surface on a metal by flame spraying itwith molten particles of-a harder metal. It is further know, as disclosed in US. Pat. No. 3,061,525, to deposit by electroplating methods a metal plate containing particles of another substance, particularly upon fiat or externally curved surfaces.

All these processes have drawbacks. Ordinary electroplating -is limited to those metals which plate out readily and which are also hard. Flame spraying is expensive and difficult to control, and requires extensive subsequent finishing operations. The process of US. Pat. NO. 3,016,525 is slow, and is limited to flat objects'on which particles can settle, or to externally curved objects which can be rotated during processing to receive settling particles. This is not only cumbersome, but does not readily provide a uniform content of solid particles per unit area.

SUMMARY OF THE INVENTION The basic achievement of the invention is a process and apparatus for producing on a workpiece, particularly apiece having internal curvature, a uniform coating comprising a metal matrix having embedded and interlocked therein dispersed particles such as various carbides or oxides,for the purpose of imparting superior wear resistance to the surface of the workpiece. It is important that good adhesion of the coating'be maintained, and that the process be rapid in order to provide high output and minimize expense.

The workpiece may first be given, by any of various wellknown processes, a light flash of zinc in the neighborhood of 1 micron thick. Then the workpiece is putinto a nickel sulfamate plating bath of the composition and under the conditions givenhereinbelow, with the surface to be" coated facing an electrode having a surface generally congruent and generally parallel to the surface to-be coated. The bath also contains minute particles of the hard material which is to form the dispersed phase of the resultant matrix.

With the bath at rat and the particles having principallysettied to the bottom, the workpiece as cathode is given a thin plate of nickel at a lowcurrent density, which will promote superior adhesion of the nickel. Since the hard particles have largely settled out, only a negligible number will adhere to the plating at this step in the process. Following this, the bath is agitated by suitable means to put the particles into suspension, and the current is increased to a high density. The particles now coming into contact with the plated surface adhere to it and are fixed in place by additional nickel ions which are now plating rapidly, thus forming a metal matrix of uniform thickness and'having a dispersed phase of hard particles to form a hard and wear-resistant coating on the workpiece.

It is therefore an object of this invention to provide a process for producing a hard and wear-resistant surface on a workpiece.

It is a further object to provide apparatus and method for producing such a surface in uniform thickness on internally curved workpieces.

Another object is to provide a method and apparatus for producing a metallic matrix having a dispersed phase of hard particles.

Other objects and advantages of the invention will become apparent on reading the following specification in connection with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is an elevational cross section of equipment and workpieces;

FIG. 2 is a plan view taken on line 22 of FIG. 1;

FIG. 3 is an elevation of a basket electrode of suitable configuration; and

FIG. 4 is a plan view of the electrode of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT An apparatus for carrying out the process is shown by way of example in the drawings. The apparatus comprises a tank 11 in which is positioned a supporting frame 12, on which are positioned a plurality of workpieces 13. In the example shown the workpieces are peripheral housings for rotary engines, it being desiredto coat their inner trochoidal surfaces 14. The housings are therefore stacked vertically in line, held apart by spacers 16 a convenient distance such as approximately a half inch, in order that the deposited coating will extend slightly over the comet edges 17 of the inner surface. The housings may be rigidly attached to the frame 12 as by bolts 18 or other suitable means to ensure that their inner surfaces are congruent.

The supporting frame 12 is pivotally supported from a bridging member 19 in such a manner that it can be swung to and fro in the bath by any suitable means (not shown). In the interiorspace of the housings, defined by the inner peripheral surfaces 14, there is positioned an inner anode 21. Anode 21 has a circumferential shape corresponding substantially to that of the surface which is to be coated, and sufficient vertical extension to protrude slightly from both top and bottom of the stack of workpieces. The anode is a titanium basket, and in the example shown has agenerally trochoidal circumference. It contains pieces of the metal to be deposited electrolytically, nickel in the present instance.

The bottom of the tank 11 may be slightly funnel shaped, and in any case has sufficient depth below the frame 12 to allowsettlin g of the dispersed particles. Extending into the bottom portion of the tank is a series of perforated tubes 22, by means of which compressed air may be blown into the bath for the purpose of keeping it in continuous agitation. There is also provided a pump 23 external to the tank, having a suction tube 24 extending into the solution, and feeding a regenerator 26 from which another tube 27 returns to the bath. By this means the bath can be recirculated for the purpose of keeping it in motion, or for regeneration if necessary.

FIGS. 3 and 4 show an example of the anode basket 21, formed preferably of titanium, which is resistant to the electrolyte. The basket has a generally epitrochoidal configuration, and may for convenience of manufacture be formed as two intersecting cylindrical sections 28 and 29, which approximate the epitrochoidal form. The basket is open at the top and can be loaded from above with metal fragments, such as nickel when that is the material to be plated out. The metal clippings or other pieces may be enclosed in a bag within the basket, which retains impurities and small bits of metal which result from electrolyte decomposition of the plating material. Such a bag is of fine mesh weave and formed of fibers which are resistant to the electrolyte used, as is common in the plating industry.

Two rodlikc conductors 31 and 32 extend from the bottom of the basket and project from the top generally parallel to the longitudinal middle axis 33 of the basket and tied together across the top of the basket by a support member 34 extending across the major diameter of the basket. The two conductors 31 and 32 have their longitudinal axes 31 and 32' positioned on the major diameter of the basket, and radially outside the longitudinal axes 28 and 29' of the two partial cylinders 28 and 29. The amount of such displacement of the electrode axes from the centers of curvature of the cylindrical sections is small, and has been exaggerated in the drawings for clarity of illustration. The amount of displacement is ordinarily from about 5 percent to about percent of the radius of curvature. The exact distance of the electrodes from the cylinder axes will depend on the shape and size of the housing surface to be coated. The two conductors are connected electrically parallel, as anode, to a source of direct current (not shown), and the positioning results in a uniform field of equipotential lines of force which ensures uniform coating of the cathodic housing surfaces.

The composition of the plating bath is as follows:

Nickel (in nickel sulfamate 80-100 grams per liter and nickel chloride) Chlorine (in nickel chloride) 2.5-3 grams per liter Boric acid 20-30 grams per liter saccharine 1.5-2.5 grams per liter Hard particles 90-l00 grams per liter The hard particles may be one of various oxides or carbides, such as silicon carbide, silicon dioxide, aluminum oxide, tungsten carbide, boron carbide, or mixtures of these substances. The grain size of the hard particle powder is preponderantly 1 micron and less, the number of particles greater than 3 microns being not more than 1 percent.

The electrode basket is filed with pieces of nickel, which may conveniently be squares or clippings. The pH value of the bath has an effect on the quantity of precipitated solid particles, and in accordance with the invention it must be above 5, preferably 5.2. The bath is maintained at a temperature of 50 to 70 C.

With the bath at rest and the particles principally settled out, plating is begun at low current density, below 90 amperes per square foot and preferably at about 18.6. Nickel plate is deposited on the housings as cathode at this low current density, the initial deposit being from 10 to 20 microns thick and preferably about microns, which takes about to 30 minutes. The low current density promotes good adhesion, and only a negligible number of solid particles is precipitated during this phase.

When the basic layer of nickel has reached the desired thickness, the bath is agitated for the purpose suspending the solid particles in the electrolyte. This may be accomplished by swinging the suspension frame 12 on its pivot, or by blowing air into the bath through tube 22, or by means of the circulating pump 23, or any combination of these. At the same time the current is increased to about 186 amperes per square foot, that is, about l0 times the initial plating rate. The hard solid particles now precipitate on the plating surface, while nickel is plating out at an increased rate, embedding and locking the particles into place. This phase is continued for about 100 minutes until the coating is from 300 to 320 microns thick. The total coating time, including initial preparation of the surface, is about 140 minutes, as compared to about 1,100 minutes for chromium plating. The time can be decreased somewhat, since the duration of the coating operation decreases with a rise in current density, which can be varied within wide limits duringthe second phase of the operation, when solid particles are being deposited. However, more rapid deposition results in some increased roughness of the surface.

The positioning of the housings on the frame 12 may be modified so that the housings are stacked contiguously, with their inner surfaces 14 forming a hollow tube. In such a case the bath may be circulated at a selected flow rate by the pump 23 through the space between the anode and the housings, with the pump discharge tube 27 emptying into the interelectrode space.

Although the invention has been described above in a preferred embodiment it is not limited thereto, but is suited to the coating of other types of workpieces, and various modifications may be made by those skilled in the art without departing from the scope of the invention.

What is claimed is:

l. A process for the electrolytic application on a workpiece of a wear-resistant coating consisting of a metal matrix containing dispersed particles of a hard solid substance, compris ing:

a. depositing a layer of zincof the order of 1 micron thick on the workpiece; then electrolytically depositing a layer of nickel from 10 to 20 microns thick on the zinc in an electrolytic nickel bath containing solid particles having a grain size of 1 micron and less in quantities of to grams per liter, while the bath is at rest and the particles are substantially settled out, and the current density is between l8 and 90 amperes per square foot;

' c. then in the same bath with a current density from to amperes per square foot and with continuous relative agitation between the bath and the workpiece to put the particles in suspension electrolytically depositing on the nickel layer a further layer from 300 to 320 microns thick consisting of a nickel matrix containing dispersed hard particles.

2. A process as recited in claim I, wherein during the deposition of the matrix containing particles the electrolyte bath is held at a pH between 5 and 6.

3. A process as recited in claim 2, wherein the electrolytic bath contains, per liter of electrolyte: 80 to 100 grams of nickel in nickel sulfamate and nickel chloride, 2.5 to 3 grams of chlorine in nickel chloride, 20 to 30 grams of boric acid, 1.5 to 2.5 grams of saccharine, and 90 to 100 grams of particles selected from the group consisting of silicon carbide, silicon dioxide, aluminum oxide, tungsten carbide, and boron car bide; and the bath is held during deposition of the matrix at a temperature of 50 to 70 C., and at a pH of 5.2.

4. A process as recited inclaim 3, wherein the first nickel layer is approximately 15 microns thick and is deposited at a current density of approximately 18.6 amperes per square foot, and the second nickel layer containing hard particles is deposited at a current density of approximately 186 amperes per square foot.

5. A process as recited in claim 4, wherein the current density of approximately 18.6 amperes per square foot is maintained for 20 to 30 minutes to deposit the first nickel layer, and the current density of approximately 186 amperes per square foot is maintained for about 100 minutes to deposit the second nickel layer containing hard particles. 

2. A process as recited in claim 1, wherein during the deposition of the matrix containing particles the electrolytic bath is held at a pH between 5 and
 6. 3. A process as recited in claim 2, wherein the electrolytic bath contains, per liter of electrolyte: 80 to 100 grams of nickel in nickel sulfamate and nickel chloride, 2.5 to 3 grams of chlorine in nickel chloride, 20 to 30 grams of boric acid, 1.5 to 2.5 grams of saccharine, and 90 to 100 grams of particles selected from the group consisting of silicon carbide, silicon dioxide, aluminum oxide, tungsten carbide, and boron carbide; and the bath is held during deposition of the matrix at a temperature of 50* to 70* C. and at a pH of 5.2.
 4. A process as recited in claim 3, wherein the first nickel layer is approximately 15 microns thick and is deposited at a current density of approximately 18.6 amperes per square foot, and the second nickel layer containing hard particles is deposited at a current density of approximately 186 amperes per square foot.
 5. A process as recited in claim 4, wherein the current density of approximately 18.6 amperes per square foot is maintained for 20 to 30 minutes to deposit the first nickel layer, and the current density of approximately 186 amperes per square foot is maintained for about 100 minutes to deposit the second nickel layer containing hard particles. 